Petrol driven internal combustion engines are normally provided with a butterfly valve which regulates the flow of air supplied to the cylinders. Typically, the butterfly valve comprises a valve body housing a valve seat engaged by a butterfly disc which is keyed on a shaft in order to rotate between an open position and a closed position under the action of an electric motor connected to this shaft by means of a gear transmission. The shaft bearing the butterfly valve is associated with a position sensor which is adapted to detect the angular position of the shaft and therefore of the butterfly valve in order to enable a control unit to control, by feedback, the electric motor which determines the position of the butterfly valve.
The butterfly valve normally comprises a torsion spiral return spring which is mounted coaxially with the shaft and is mechanically coupled to the shaft in order to exert a torque on this shaft which tends to bring the shaft towards the closed position; and a torsion spiral opposing spring which is mounted coaxially with the shaft and is mechanically coupled to the shaft in order to exert a torque on this shaft which tends to bring the shaft into a partially open position (called the limp-home position) against the action of the return spring and as a result of the presence of an abutment body which defines an abutment for the opposing spring against which the opening movement determined by this opposing spring is stopped. The torque generated by the opposing spring is greater than the torque generated by the return spring; for this reason, when the motor is not activated the shaft is disposed in the limp-home position and the motor itself then has to generate a respective drive torque both to bring the shaft into the position of maximum opening and to bring the shaft into the closed position.
US20020129791 discloses a throttle device for an internal-combustion engine, in which, on one side of the side wall of a throttle body, there are formed a space for mounting a reduction gear mechanism which transmits the power from a motor to a throttle valve shaft and a default opening setting mechanism for holding a throttle valve opening at a specific opening (default opening) when the ignition switch is in off position, and a gear cover mounting frame which edges the mounting space; the frame is formed lower than the mounting level of the reduction gear mechanism. A gear cover for covering the gear mounting space is attached on the frame; a stopper for defining the default opening and a stopper for defining the full-closed position of the throttle valve are juxtaposed so as to enable position adjustments in the same direction. These stoppers serve to stop a default lever and a throttle gear, thereby enabling downsizing, weight reduction, and rationalization of fabrication and adjustments of an electronically controlled throttle device.
The solution described above in which the limp-home position is established by two spiral springs is normally used in the butterfly valves available commercially; however, this solution has some drawbacks as it is very bulky and relatively complex and time-consuming to assemble.